Help with contrast/brightness on HDMI output

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  • May 11, 2021 at 3:52 AM #47273
    Doug
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    Hi folks,

    I got an OSSC recently to help with a 1990 vintage computer, with 3 BNC RGB outputs which put out a Sync-on-green signal at 1280×1024 and 60Hz. Via a BNC-to-VGA cable, I can display the output just fine on (for example) a Dell 2009W LCD (native resolution 1680×1050, so some scaling artifacts). I am trying to convert the analog output to HDMI so I can use an HDMI capture system that I have.

    I am noticing problems getting the HDMI conversion on the OSSC to have a nice, smooth brightness ramp. What I mean is, when I output a test pattern (e.g., a 20-bar color gradient of RGB, CMY and Gray), directly connecting the VGA to the Dell, I can see 18-20 of the 20 colors, with a little tweaking of brightness and contrast. However, when I view the DVI output from the OSSC on the same monitor, I can usually only discern about 12-14. The darker colors are all clipped to black, and the two or three brightest colors are indistinguishable.

    I have tweaked the R/G/B offset and gain, and the Pre-ADC gain a lot, but don’t seem to be making much headway. The offsets look best at 0, and the gains anywhere between 25-75, and a pre-AGC gain around 5. Any ideas on what to try please?

    I am using OSSC v1.6 with firmware 0.81a, with the VGA input from my 3BNC outputs, and the HDMI output to a DVI monitor (I do set the DVI mode) or HDMI capture device (AV.io HD; I do set the HDMI mode).

    (Also, is there a way to get the OSSC to output a 1280×1024 digital signal? It is outputting a 1080p signal.)

    Thanks!

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